Can i use this on any smart tv or does it have to be Apple TV?

Fitness+ is available on iPhone, iPad, and Apple TV. You can certainly connect your Apple TV to your smart TV, or if your TV supports AirPlay, use that to mirror your iPhone or iPad to the TV to see your Fitness+ workout.

How to use Fitness+

Note that if you mirror Apple Fitness to a smart TV, you won't see the burn bar.